Paul Unworthy to be Called an Apostle, and a Liar

I have heard this characterization of Paul, as a reason why we should not trust Paul's writings:

⁹ For I am the least of the apostles, unworthy to be called an apostle..
(1 Cor 15:9)

As mentioned elsewhere Paul did not show up in Corinth with a big stick and force everyone by threat of death to convert to Christianity.  Nor did he go around saying "I am a prophet of God so do what I say".

Instead, he presents humility and [sometimes tedious] reasoning in his writing.  
This begs the question how he ever made any converts at all.

The verse below is also used to show Paul is a liar, but not a very clever one:

⁷ But if by my lie God’s truth is amplified to His glory, why am I also still judged as a sinner?
(Romans 3:7)